One well-known method of growing crystals, the Czochralski method, involves dipping a crystal seed into a quantity of semiconductor material, and withdrawing the seed slowly so as to produce a resultant crystal of predetermined shape and size. The semiconductor material is kept in a quartz crucible, and typically is heated by an onboard heater so as to bring the material to a molten state. Temperature of the melt is critical, such temperature being determinative of the resultant crystal's shape and size.
Another important factor to the crystal-growing process is the oxygen content of the resultant crystal, particularly where the crystal is formed of a material such as silicon. It will be appreciated, for example, that dissolution of silica (SiO.sub.2) during crystal growth introduces significant amounts of oxygen into the melt and thus into the growing crystal. Although traditional applications allow for relatively high amounts of oxygen in the crystal without ill effect, many current semiconductor applications call for crystals which have lower oxygen concentrations. Accordingly, it would be desirable to provide an system whereby the oxygen concentration of grown crystals may be controlled.
As will be appreciated by those skilled in the art, the amount of oxygen in a crystal grown by the Czochralski method is determined by the natural thermal convection of oxygen to the melt surface, and by the thermal characteristics of the heater which heats the melt. It will be noted, for example, that a conventional heater defines an elongate heat zone which surrounds the crucible to uniformly heat the melt. This tends to direct oxygen toward the melt surface, and thus tends to increase oxygen in the growing crystal, which is drawn from the surface of the melt. The present invention arises from recognition of the benefit of shifting the heat balance of the system's heater so as to suppress the natural thermal melt convections which transport oxygen without significantly detracting from the heater's ability to control the temperature of the melt.